Lithium-ion secondary batteries and separation membranes

Abstract

One aspect of the present invention provides a lithium-ion secondary battery comprising, in sequence, a positive electrode mixture layer, a separator, and a negative electrode mixture layer, wherein the positive electrode mixture layer contains a positive electrode active material, a first lithium salt, and a first solvent; the negative electrode mixture layer contains a negative electrode active material, a second lithium salt, and a second solvent different from the first solvent; and the separator contains at least one resin selected from the group consisting of a resin containing at least one monomer having a (meth)acryloyl group as a monomer unit and a resin containing at least one fluorine-containing olefin as a monomer unit.

Description

Technical Field

[0001] The present invention relates to a lithium ion secondary battery and a separation membrane. Background Art

[0002] In recent years, with the popularity of portable electronic devices, electric vehicles, etc., the performance requirements of secondary batteries represented by lithium-ion secondary batteries have been further improved. For example, research is underway to improve the performance of lithium-ion secondary batteries by including different types of electrolytes in the positive and negative electrodes (for example, Patent Document 1).

[0003] Previous technical literature

[0004] Patent Literature

[0005] Patent Document 1: Japanese Patent Application Laid-Open No. 2001-110447 Summary of the Invention

[0006] Technical issues to be solved by the invention

[0007] In lithium-ion secondary batteries containing different types of electrolytes at the positive and negative electrodes, it is important that the solvent contained in the electrolyte is fully separated and prevented from mixing between the positive and negative electrodes. The present inventors considered placing a separator membrane between the positive and negative electrodes to separate the solvent from the electrolyte in such lithium-ion secondary batteries. This separator membrane is required to allow lithium ions to pass through it, but it is difficult for the solvent to pass through.

[0008] An object of the present invention is to provide a separator having excellent ability to separate solvents for use in a lithium ion secondary battery containing different solvents in the positive electrode mixture layer and the negative electrode mixture layer, and a lithium ion secondary battery including the separator.

[0009] Means for solving technical problems

[0010] The present inventors have discovered that the solvent contained in the positive electrode mixture layer and the negative electrode mixture layer can be effectively separated by a separation membrane containing a specific resin, and have completed the present invention.

[0011] One aspect of the present invention provides a lithium-ion secondary battery comprising, in sequence, a positive electrode mixture layer, a separator, and a negative electrode mixture layer, wherein the positive electrode mixture layer contains a positive electrode active material, a first lithium salt, and a first solvent; the negative electrode mixture layer contains a negative electrode active material, a second lithium salt, and a second solvent different from the first solvent; and the separator contains at least one resin selected from the group consisting of a resin containing at least one monomer having a (meth)acryloyl group as a monomer unit and a resin containing at least one fluorine-containing olefin as a monomer unit.

[0012] Another aspect of the present invention provides a separation membrane, which is a separation membrane for being arranged between the positive electrode mixture layer and the negative electrode mixture layer in a lithium-ion secondary battery having a positive electrode mixture layer and a negative electrode mixture layer, the positive electrode mixture layer containing a positive electrode active material, a first lithium salt and a first solvent, the negative electrode mixture layer containing a negative electrode active material, a second lithium salt and a second solvent different from the first solvent, the separation membrane containing at least one resin selected from the group consisting of a resin containing at least one monomer having a (meth)acryloyl group as a monomer unit and a resin containing at least one fluorine-containing olefin as a monomer unit.

[0013] The separation membrane may contain a porous body and a resin held in the porous body. In this case, the porous body is preferably formed of a polymer. The separation membrane may further contain inorganic oxide particles held in the porous body.

[0014] The separation membrane may further contain a third lithium salt and a third solvent.

[0015] Effects of the Invention

[0016] The present invention can provide a separator having excellent ability to separate solvents for use in a lithium ion secondary battery containing different solvents in the positive electrode mixture layer and the negative electrode mixture layer, and a lithium ion secondary battery including the separator. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 It is a perspective view showing a lithium-ion secondary battery according to one embodiment.

[0018] Figure 2 Yes Figure 1 1 is an exploded perspective view of one embodiment of an electrode group in a lithium-ion secondary battery shown in FIG.

[0019] Figure 3 This is a schematic cross-sectional view showing one embodiment of a separation membrane.

[0020] Figure 4 This is a schematic cross-sectional view showing another embodiment of the separation membrane. DETAILED DESCRIPTION

[0021] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings as appropriate. However, the present invention is not limited to the following embodiments.

[0022] In this specification, a (meth)acryloyl group refers to an acryloyl group or a methacryloyl group corresponding thereto. The same applies to other similar expressions such as (meth)acrylate.

[0023] Figure 1 1 is a perspective view showing a lithium ion secondary battery according to one embodiment. Figure 1As shown, a lithium-ion secondary battery 1 according to one embodiment is a so-called laminated secondary battery, comprising an electrode group 2 and a pouch-shaped battery housing 3 that houses the electrode group 2. A positive electrode current collector sheet 4 and a negative electrode current collector sheet 5 are provided within the electrode group 2. The positive electrode current collector sheet 4 and the negative electrode current collector sheet 5 protrude from the interior of the battery housing 3 to the exterior, respectively, to allow the positive electrode current collector and the negative electrode current collector (described in detail later) to be electrically connected to the exterior of the lithium-ion secondary battery 1. In another embodiment, the lithium-ion secondary battery 1 may have a shape other than a laminated type (such as a coin-shaped or cylindrical shape).

[0024] The battery exterior 3 may be a container formed of a laminated film, for example, a polymer film such as polyethylene terephthalate (PET) film, a metal foil such as aluminum, copper, or stainless steel, and a sealant layer such as polypropylene laminated in this order.

[0025] Figure 2 Yes Figure 1 FIG. 1 is an exploded perspective view of an embodiment of an electrode group 2 in a lithium-ion secondary battery 1. Figure 2 As shown, the electrode assembly 2 according to this embodiment includes, in this order, a positive electrode 6, a separator 7, and a negative electrode 8. The positive electrode 6 includes a positive electrode current collector 9 and a positive electrode mixture layer 10 provided on the positive electrode current collector 9. A positive electrode current collector sheet 4 is provided on the positive electrode current collector 9. The negative electrode 8 includes a negative electrode current collector 11 and a negative electrode mixture layer 12 provided on the negative electrode current collector 11. A negative electrode current collector sheet 5 is provided on the negative electrode current collector 11.

[0026] The positive electrode current collector 9 is formed of, for example, aluminum, titanium, stainless steel, nickel, calcined carbon, conductive polymer, conductive glass, etc. The thickness of the positive electrode current collector 9 may be, for example, 1 μm or more, or 50 μm or less.

[0027] The negative electrode current collector 11 is made of, for example, copper, stainless steel, nickel, aluminum, titanium, calcined carbon, conductive polymer, conductive glass, aluminum-cadmium alloy, etc. The thickness of the negative electrode current collector 11 can be, for example, 1 μm or more or 50 μm or less.

[0028] In one embodiment, the positive electrode mixture layer 10 contains a positive electrode active material, a lithium salt (first lithium salt), and a solvent (first solvent).

[0029] The positive electrode active material may be, for example, lithium oxide. Examples of lithium oxide include Li x CoO2、Li x NiO2、Li x MnO2、Li x Co y Ni 1-y O2、Li x Co yM 1-y O z 、Li x Ni 1-y M y O z 、Li x Mn2O4 and Li x Mn 2-y M y O4 (In each formula, M represents at least one element selected from the group consisting of Na, Mg, Sc, Y, Mn, Fe, Co, Cu, Zn, Al, Cr, Pb, Sb, V, and B (wherein M is an element different from the other elements in each formula). x = 0 to 1.2, y = 0 to 0.9, z = 2.0 to 2.3.) Li x Ni 1-y M y O z The lithium oxide represented can be Li x Ni 1-(y1+y2) Co y1 Mn y2 O z (where x and z are the same as above, y1=0-0.9, y2=0-0.9, and y1+y2=0-0.9.) For example, LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2、LiNi 0.5 Co 0.2 Mn 0.3 O2、LiNi 0.6 Co 0.2 Mn 0.2 O2、LiNi 0.8 Co 0.1 Mn 0.1 O2.Li x Ni 1-y M y O z The lithium oxide represented can be Li x Ni 1-(y3+y4) Co y3 Al y4 O z (where x and z are the same as above, y3 = 0 to 0.9, y4 = 0 to 0.9, and y3 + y4 = 0 to 0.9.) For example, LiNi 0.8 Co 0.15 Al 0.05 O2.

[0030] The positive electrode active material may be a lithium phosphate. Examples of lithium phosphates include lithium manganese phosphate (LiMnPO4), lithium iron phosphate (LiFePO4), lithium cobalt phosphate (LiCoPO4), and lithium vanadium phosphate (Li3V2(PO4)3). The above-mentioned positive electrode active materials may be used alone or in combination of two or more.

[0031] The content of the positive electrode active material, based on the total amount of the positive electrode mixture layer, may be 70% by mass or more, 80% by mass or more, or 85% by mass or more. The content of the positive electrode active material, based on the total amount of the positive electrode mixture layer, may be 95% by mass or less, 92% by mass or less, or 90% by mass or less.

[0032] The first lithium salt can be, for example, at least one selected from the group consisting of LiPF6, LiBF4, LiClO4, LiNO3, LiB(C6H5)4, LiCH3SO3, CF3SO2OLi, LiN(SO2F)2 (LiFSI, lithium bis(fluoromethanesulfonyl)imide), LiN(SO2CF3)2 (LiTFSI, lithium bis(trifluoromethanesulfonyl)imide) and LiN(SO2CF2CF3)2.

[0033] The content of the first lithium salt may be 0.5 mol / L or more, 0.7 mol / L or more, or 0.8 mol / L or less, or 1.5 mol / L or less, 1.3 mol / L or less, or 1.2 mol / L or less, based on the total amount of the first solvent.

[0034] The first solvent is a solvent for dissolving the first lithium salt. Examples of the first solvent include cyclic carbonates such as ethylene carbonate, propylene carbonate, vinylene carbonate, vinylethylene carbonate, fluoroethylene carbonate, and difluoroethylene carbonate; chain carbonates such as dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate; cyclic esters such as γ-butyrolactone, γ-valerolactone, δ-valerolactone, ε-caprolactone, and γ-valerolactone; tetrahydrofuran, 1,3-dioxane, dimethoxyethane, diethoxyethane, methoxyethoxyethane; ethers such as ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether; phosphoric acid esters such as triesters; nitriles such as acetonitrile, benzonitrile, adiponitrile, and glutaronitrile; chain sulfones such as dimethyl sulfone and diethyl sulfone; cyclic sulfones such as sulfolane; and cyclic sulfonates such as propane sultone. The first solvent can be used alone or in combination of two or more.

[0035] The first solvent is preferably an anti-oxidizing solvent such as acetonitrile or ethylene carbonate, which can improve the anti-oxidizing property of the positive electrode mixture layer 10 .

[0036] The content of the first solvent contained in the positive electrode mixture layer 10 can be appropriately set within a range capable of dissolving the first lithium salt, and can be, for example, 10% by mass or more and 80% by mass or less based on the total amount of the positive electrode mixture layer.

[0037] The positive electrode mixture layer 10 may further contain a binder and a conductive material as other components.

[0038] The binder may be a polymer containing at least one monomer unit selected from the group consisting of tetrafluoroethylene, vinylidene fluoride, hexafluoropropylene, acrylic acid, maleic acid, ethyl methacrylate, methyl methacrylate, and acrylonitrile, or a rubber such as styrene-butadiene rubber, isoprene rubber, or acrylic rubber. The binder is preferably polyvinylidene fluoride or a copolymer containing hexafluoropropylene and vinylidene fluoride as monomer units.

[0039] The binder content can be 0.3 mass %, 0.5 mass %, 1 mass % or 1.5 mass % or more, based on the total amount of the positive electrode mixture layer, and can also be 10 mass %, 8 mass %, 6 mass % or 4 mass % or less.

[0040] The conductive material may be carbon materials such as carbon black, acetylene black, graphite, carbon fiber, carbon nanotube, etc. These conductive materials may be used alone or in combination of two or more.

[0041] The content of the conductive material, based on the total amount of the positive electrode mixture layer, may be 0.1% by mass or more, 1% by mass or more, or 3% by mass or more. From the perspective of increasing the volume of the positive electrode 6 and the resulting decrease in the energy density of the lithium-ion secondary battery 1, the content of the conductive material, based on the total amount of the positive electrode mixture layer, is preferably 15% by mass or less, more preferably 10% by mass or less, and even more preferably 8% by mass or less.

[0042] The thickness of the positive electrode mixture layer 10 may be 5 μm or more, 10 μm or more, 15 μm or more, or 20 μm or less, or 100 μm or less, 80 μm or less, 70 μm or less, or 50 μm or less.

[0043] In one embodiment, the negative electrode mixture layer 12 contains a negative electrode active material, a lithium salt (second lithium salt), and a solvent (second solvent).

[0044] The negative electrode active material can use materials commonly used in the field of energy devices. Specifically, examples of the negative electrode active material include metallic lithium, lithium titanate (Li4Ti5O 12), lithium alloys or other metal compounds, carbon materials, metal complexes, organic polymer compounds, etc. These negative electrode active materials can be used alone or in combination of two or more. As carbon materials, natural graphite (such as flaky graphite), graphite such as artificial graphite (graphite), amorphous carbon, carbon fiber, and carbon black such as acetylene black, stove black, channel black, furnace black, lamp black, thermal black, etc. can be mentioned. From the viewpoint of obtaining a larger theoretical capacity (for example, 500 to 1500 Ah / kg), the negative electrode active material can be a negative electrode active material containing silicon as a constituent element, a negative electrode active material containing tin as a constituent element, etc. Among these, the negative electrode active material can be a negative electrode active material containing silicon as a constituent element.

[0045] The negative electrode active material containing silicon as a constituent element may be an alloy containing silicon as a constituent element, for example, an alloy containing silicon and at least one element selected from the group consisting of nickel, copper, iron, cobalt, manganese, zinc, indium, silver, titanium, germanium, bismuth, antimony, and chromium. The negative electrode active material containing silicon as a constituent element may be an oxide, nitride, or carbide. Specifically, examples include silicon oxides such as SiO, SiO2, and LiSiO; silicon nitrides such as Si3N4 and Si2N2O; and silicon carbides such as SiC.

[0046] The content of the negative electrode active material, based on the total amount of the negative electrode mixture layer, may be 60% by mass or more, 65% by mass or more, or 70% by mass or more. The content of the negative electrode active material, based on the total amount of the negative electrode mixture layer, may be 99% by mass or less, 95% by mass or less, or 90% by mass or less.

[0047] The type and content of the second lithium salt may be the same as those of the first lithium salt contained in the positive electrode mixture layer 10. The second lithium salt may be of the same type as or different from the first lithium salt.

[0048] The second solvent is a solvent for dissolving the second lithium salt. The second solvent can be the same solvent as the first solvent described above, but a different solvent can be used. This allows for the use of solvents suitable for the positive electrode 6 and the negative electrode 8, respectively, thereby improving various properties of the lithium-ion secondary battery 1, such as energy density and life.

[0049] The second solvent is preferably a solvent having reduction resistance, such as γ-butyrolactone or tetrahydrofuran, so that the reductive decomposition of the second solvent contained in the negative electrode mixture layer 12 can be suppressed.

[0050] The content of the second solvent contained in the negative electrode mixture layer 12 can be appropriately set within a range capable of dissolving the second lithium salt, and can be, for example, 10% by mass or more and 80% by mass or less based on the total amount of the negative electrode mixture layer.

[0051] The negative electrode mixture layer 12 may further contain a binder and a conductive material as other components. The types and contents of the binder and the conductive material may be the same as those in the positive electrode mixture layer 10 described above.

[0052] The thickness of the negative electrode mixture layer 12 may be 10 μm or more, 15 μm or more, or 20 μm or less, or 100 μm or less, 80 μm or less, 70 μm or less, 50 μm or less, 40 μm or less, or 30 μm or less.

[0053] In the lithium-ion secondary battery 1, the separator 7 is disposed between the positive electrode mixture layer 10 and the negative electrode mixture layer 12. The separator 7 separates the first solvent and the second solvent contained in the positive electrode mixture layer 10 and the negative electrode mixture layer 12 from each other to prevent them from mixing. Lithium ions can be transferred through the separator 7.

[0054] The separation membrane 7 contains at least one resin selected from the group consisting of a resin containing at least one monomer having a (meth)acryloyl group represented by the following formula (1) as a monomer unit (hereinafter also referred to as an "acrylic resin") and a resin containing at least one fluorine-containing olefin as a monomer unit (hereinafter also referred to as a "fluororesin"). The separation membrane 7 may contain only this resin or may contain this resin and other components.

[0055] Figure 3 This is a schematic cross-sectional view showing one embodiment of a separation membrane 7. This separation membrane 7A contains: at least one resin selected from the group consisting of a resin containing as a monomer unit at least one of monomers having a (meth)acryloyl group and a resin containing as a monomer unit at least one of fluorine-containing olefins; a lithium salt (third lithium salt); and a solvent (third solvent).

[0056] The acrylic resin may be a homopolymer containing as monomer units only one of the monomers having a (meth)acryloyl group represented by the following formula (1), or a copolymer containing as monomer units two or more of the monomers. The content of the monomer having a (meth)acryloyl group may be 70% by mass or more, 80% by mass or more, or 90% by mass or more, based on the total amount of monomer units contained in the acrylic resin. The acrylic resin may be composed solely of monomer units having a (meth)acryloyl group.

[0057]

[0058] In formula (1), R 1 represents a hydrogen atom or a methyl group, and * represents a bond.

[0059] Examples of the acrylic resin include polyalkyl (meth)acrylates such as polymethyl methacrylate; poly(polyalkylene glycol di(meth)acrylates) such as poly(polyethylene glycol di(meth)acrylate); and poly(meth)acrylic acid.

[0060] A fluororesin is a resin containing at least one fluorine-containing olefin as a monomer unit. A fluororesin may be a homopolymer containing only one fluorine-containing olefin as a monomer unit, or a copolymer containing two or more fluorine-containing olefins as monomer units. The content of the fluorine-containing olefin, i.e., monomer units, may be 70% by mass or greater, 80% by mass or greater, or 90% by mass or greater, based on the total amount of monomer units contained in the fluororesin. A fluororesin may be composed solely of fluorine-containing olefin, i.e., monomer units.

[0061] Examples of the fluororesin include polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF).

[0062] As the third lithium salt, the same lithium salt as the first lithium salt can be used. The third lithium salt may be of the same type as the first and second lithium salts, or may be different.

[0063] The content of the third lithium salt may be 1 mass % or more, 1.5 mass % or more, or 2 mass % or less, or 30 mass % or less, 25 mass % or less, or 20 mass % or less, based on the total amount of the separation membrane.

[0064] The third solvent is used to dissolve the third lithium salt and also has the function of further improving the lithium ion conductivity of separation membrane 7A. The same solvents as the first solvent described above can be used as the third solvent. The third solvent may be of the same type as the first and second solvents, or may be different.

[0065] The content of the third solvent may be 1 mass % or more, 3 mass % or more, or 5 mass % or less, or 60 mass % or less, 55 mass % or less, or 50 mass % or less, based on the total amount of the separation membrane.

[0066] From the perspective of further improving the separation capability of separation membrane 7A, the thickness of separation membrane 7A is preferably 100 μm or greater, 200 μm or greater, or 500 μm or greater. From the perspective of increasing the energy density of lithium-ion secondary battery 1, the thickness of separation membrane 7 is preferably 800 μm or less, 600 μm or less, or 400 μm or less.

[0067] Since the separation membrane 7A contains the resin and lithium salt, it has lithium ion conductivity. Having lithium ion conductivity means that in the presence of a lithium salt, it has the property of being able to conduct lithium ions derived from the lithium salt. Whether the separation membrane 7A can conduct lithium ions can be confirmed by measuring the ion conductivity of the separation membrane 7A. If the peak of the ion conductivity measured when 1 to 40% by mass of lithium salt is added to the separation membrane 7A is 1×10 -6 S / cm or more, it can be said to have lithium ion conductivity.

[0068] The separation membrane 7A can be produced, for example, by the following method. Specifically, the method comprises forming a slurry containing at least one monomer selected from the group consisting of a monomer capable of forming a resin containing at least one of the aforementioned monomers having a (meth)acryloyl group as a monomer unit (hereinafter referred to as an "acrylic monomer") and a monomer capable of forming a resin containing at least one of the aforementioned fluorine-containing olefins as a monomer unit (hereinafter referred to as a "fluorine monomer"), a third lithium salt, and a third solvent into a film-like shape, and then polymerizing the monomer.

[0069] The slurry can be formed into a film by, for example, placing a frame of any size on one surface of a substrate such as a PET sheet and pouring the slurry into the frame. Alternatively, the film can be formed by applying the monomer to one surface of the substrate using a doctor blade method, a dipping method, a spraying method, or the like.

[0070] In order to polymerize the monomers, a polymerization initiator may be added to the slurry. The polymerization initiator may be a thermal polymerization initiator or a photopolymerization initiator.

[0071] Examples of the thermal polymerization initiator include azo compounds such as azobisisobutyronitrile and azobis(2-methylbutyronitrile).

[0072] Examples of the photopolymerization initiator include 2-hydroxy-2-methyl-1-phenylpropanol and diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide.

[0073] When a polymerization initiator is added, the content of the polymerization initiator can be 0.01 parts by mass or more, 0.05 parts by mass or more, 0.1 parts by mass or more, 1 part by mass or more, 5 parts by mass or more, 10 parts by mass or more, or 20 parts by mass or less, or 50 parts by mass or less, or 40 parts by mass or less, or 30 parts by mass or less, or 20 parts by mass or less, or 15 parts by mass or less, or 10 parts by mass or less, relative to 100 parts by mass of the monomer.

[0074] In one embodiment, the monomer is polymerized by applying heat under predetermined conditions. The heating temperature is, for example, 50 to 90° C. The heating time can be appropriately adjusted depending on the heating temperature, for example, 1 minute to 2 hours.

[0075] In another embodiment, the method for polymerizing the monomer is a method of irradiating light under predetermined conditions. In one embodiment, the monomer can be polymerized by irradiating light having a wavelength within the range of 200 to 400 nm (ultraviolet light).

[0076] Figure 4 (a) and Figure 4 (b) is a schematic cross-sectional view showing another embodiment of the separation membrane 7. This separation membrane 7B, 7C comprises a porous body 21 and a resin 22 retained by the porous body 21. The porous body 21 has a porous structure with fine pores. The use of the porous body 21 facilitates the retention of the resin 22 and further enhances the solvent separation capability.

[0077] exist Figure 4 In the separation membrane 7B shown in (a), the resin 22 covers the surface of the porous body 21, so that the resin 22 is retained in the porous body 21. The resin 22 preferably covers the entire surface of the porous body 21.

[0078] exist Figure 4 In the separation membrane 7C shown in (b), the resin 22 is impregnated into the porous body 21, so that the resin 22 is retained in the porous body 21. The resin 22 exists in the pores of the porous body 21.

[0079] The porous body 21 is preferably formed of a polymer. The polymer forming the porous body 21 is not particularly limited as long as it is a polymer other than the above-mentioned acrylic resin and fluororesin. Examples of the polymer forming the porous body 21 include polyalkylene glycols such as polyethylene glycol, polypropylene glycol, and polytetramethylene ether glycol, polyolefins, and polyvinyl acetate.

[0080] From the perspective of easily retaining the resin 22, the Gurley value of the porous body 21 is preferably 20 seconds / 100cc or less, more preferably 10 seconds / 100cc or less, and even more preferably 5 seconds / 100cc or less. The Gurley value of the porous body 21 can be, for example, 1 second / 100cc or more. The Gurley value of the porous body 21 can be calculated as air permeability (seconds / 100cc) by measuring the time it takes for 100cc of air to pass through the porous body 21 using a Gurley densitometer (e.g., No. 323 manufactured by Yasuda Seiki Seisakusho, Ltd.).

[0081] From the perspective of easily retaining the resin 22, the porosity of the porous body 21 is preferably 30% by volume or more, more preferably 40% by volume or more, and even more preferably 50% by volume or more, based on the volume of the porous body 21. From the perspective of maintaining the strength of the porous body, for example, the porosity of the porous body 21 is preferably 90% by volume or less, more preferably 80% by volume or less, and even more preferably 70% by volume or less, based on the volume of the porous body 21. The porosity of the porous body 21 can be calculated using the following formula: volume a obtained from the weight and true density of the porous body, and volume b obtained by measuring the area and thickness of the porous body.

[0082] Porosity = (1-a / b) × 100

[0083] The resin 22 is at least one resin selected from the group consisting of resins containing at least one of the aforementioned monomers having a (meth)acryloyl group as a monomer unit (acrylic resin) and resins containing at least one of fluorine-containing olefins as a monomer unit (fluororesin).

[0084] In the separation membranes 7B and 7C, from the perspective of further improving the separation capability of the separation membrane 7, the content of the resin 22 retained in the porous body 21 is preferably 20% by volume or more, more preferably 30% by volume or more, and even more preferably 40% by volume or more, based on the total volume of the separation membrane. From the perspective of maintaining the strength of the separation membrane, the content of the resin 22 retained in the porous body 21 is preferably 90% by volume or less, more preferably 85% by volume or less, and even more preferably 80% by volume or less, based on the total volume of the separation membrane.

[0085] Separation membranes 7B and 7C may further contain inorganic oxide particles. This can further improve the ion conductivity of separation membranes 7B and 7C. In one embodiment, when separation membrane 7B contains inorganic oxide particles, the composition comprising the resin 22 and inorganic oxide particles is coated on the surface of porous body 21. In one embodiment, when separation membrane 7C contains inorganic oxide particles, the composition comprising the resin 22 and inorganic oxide particles is impregnated into porous body 21.

[0086] Inorganic oxide particles can be, for example, Li2O, Al2O3, TiO2, GeO2, SiO2, P2O5, Li7La3Zr2O 12 The main crystalline phase of the inorganic oxide particles can be Li 1+x+y Al x Ti 2-x Si y P3- y O 12(0≤x≤1, 0≤y≤1, preferably 0≤x≤0.4, 0<y≤0.6, more preferably 0.1≤x≤0.3, 0.1<y≤0.4.) The inorganic oxide particles may be used alone or in combination of two or more.

[0087] The average particle size of the inorganic oxide particles may be 2 μm or more, 10 μm or more, or 50 μm or more, or 250 μm or less, 180 μm or less, or 100 μm or less. The average particle size of the inorganic oxide particles is measured by measuring the particle size distribution using a laser diffraction particle size distribution analyzer.

[0088] From the perspective of further improving the ion conductivity of the separation membranes 7B and 7C, the content of the inorganic oxide particles is preferably 0.1% by volume or more, more preferably 0.3% by volume or more, and even more preferably 0.5% by volume or more, based on the total volume of the separation membrane. From the perspective of further improving the separation capacity of the separation membrane 7, the content of the inorganic oxide particles is preferably 20% by volume or less, more preferably 10% by volume or less, and even more preferably 5% by volume or less.

[0089] The separation membranes 7B and 7C may contain a third lithium salt and a third solvent as other components. Specific embodiments of the third lithium salt and the third solvent are as described above. In one embodiment, the surface of the porous body 21 is coated with a composition comprising the resin 22, the third lithium salt, and the third solvent.

[0090] When the separation membranes 7B and 7C contain the third lithium salt, the content of the third lithium salt can be greater than 1 mass %, greater than 1.5 mass % or greater than 2 mass %, or less than 30 mass %, less than 25 mass % or less than 20 mass % based on the total amount of the separation membrane.

[0091] When the separation membranes 7B and 7C contain the third solvent, the content of the third solvent can be greater than 1 mass%, greater than 3 mass% or greater than 5 mass%, or less than 60 mass%, less than 55 mass% or less than 50 mass%, based on the total amount of the separation membrane.

[0092] The thickness of the separation membranes 7B and 7C may be within the same range as the thickness of the separation membrane 7A.

[0093] Since separation membranes 7B and 7C contain the aforementioned resin (acrylic resin and / or fluororesin), they have lithium ion conductivity. The method for confirming the lithium ion conductivity is as described above.

[0094] The separation membranes 7B and 7C can be manufactured, for example, by the following method. Specifically, the method for manufacturing the separation membranes 7B and 7C includes the following steps: preparing a porous body 21, holding a slurry containing a monomer capable of forming a resin 22 (at least one monomer selected from the group consisting of a monomer capable of forming a resin containing at least one monomer having a (meth)acryloyl group as a monomer unit and a monomer capable of forming a resin containing at least one fluorine-containing olefin as a monomer unit) in the porous body 21, and polymerizing the monomer in the slurry.

[0095] The porous body 21 having the above-mentioned properties can be produced by a known method, or a commercially available product having the above-mentioned properties can be prepared.

[0096] The slurry contains a monomer capable of forming the resin 22. The slurry may contain only the monomer. When the separation membranes 7B and 7C contain inorganic oxide particles, a third lithium salt, and a third solvent, the slurry may further contain these.

[0097] The slurry may further contain a polymerization initiator. This allows the monomers in the slurry to be properly polymerized, and separation membranes 7B and 7C to be properly produced. The polymerization initiator may be a thermal polymerization initiator or a photopolymerization initiator, and can be appropriately selected depending on the intended purpose. The polymerization initiator may be the same as the polymerization initiator used for separation membrane 7A described above.

[0098] The content of the polymerization initiator can be 0.5% by mass or more, 1% by mass or more, 5% by mass or more, 10% by mass or more, or 20% by mass or less, or 50% by mass or less, or 40% by mass or less, or 30% by mass or less, or 10% by mass or less, or 5% by mass or less, or 3% by mass or less, based on the total amount of the slurry.

[0099] Regarding the method for retaining the slurry on the porous body 21, for example, when obtaining the separation membrane 7B, the slurry may be applied to the surface of the porous body 21. The application method may be a doctor blade method, a dipping method, a spraying method, etc. In this case, it is preferable to apply the slurry to the entire surface of the porous body 21.

[0100] The porous body 21 may be held in the slurry by, for example, immersing the porous body 21 in the slurry when the separation membrane 7C is obtained. For example, the porous body 21 may be immersed in the slurry for 1 to 10 minutes. This allows the slurry to penetrate into the pores of the porous body 21.

[0101] Then, the monomers in the slurry are polymerized to obtain separation membranes 7B and 7C in which the resin 22 is retained by the porous body 21 .

[0102] Regarding the method for polymerizing the monomers, when the slurry contains a thermal polymerization initiator, heat is applied under predetermined conditions to the porous body 21 holding the slurry. The heating temperature and heating time may be the same as those in the method for producing the separation membrane 7A.

[0103] The method for polymerizing the monomers is to irradiate the porous body 21 holding the slurry with light under predetermined conditions when the slurry contains a photopolymerization initiator. The light irradiation method may be the same as the method for producing the separation membrane 7A described above.

[0104] Next, a method for manufacturing a lithium-ion secondary battery 1 will be described. The method for manufacturing a lithium-ion secondary battery 1 according to one embodiment includes: obtaining a positive electrode 6 having a positive electrode mixture layer 10 containing a positive electrode active material, a first lithium salt, and a first solvent; obtaining a negative electrode 8 having a negative electrode mixture layer 12 containing a negative electrode active material, a second lithium salt, and a second solvent different from the first solvent; and providing a separator 7 between the positive electrode 6 and the negative electrode 8. The order of the steps is arbitrary.

[0105] In the above-mentioned production method, specific aspects of the positive electrode active material, the first lithium salt, the first solvent, the negative electrode active material, the second lithium salt, the second solvent, and the separator 7 are as described above.

[0106] In the steps of obtaining the positive electrode and obtaining the negative electrode, the positive electrode 6 and the negative electrode 8 can be obtained using known methods. For example, a kneader, disperser, or the like is used to disperse the materials used for the positive electrode mixture layer 10 or the negative electrode mixture layer 12 in an appropriate amount of a dispersion medium to obtain a slurry of the positive electrode mixture or the negative electrode mixture. The positive electrode mixture or the negative electrode mixture is then applied to the positive electrode collector 9 or the negative electrode collector 11 using a doctor blade method, a dipping method, a spraying method, or the like, and the dispersion medium is evaporated to obtain the positive electrode 6 and the negative electrode 8. In this case, the dispersion medium can be water, N-methyl-2-pyrrolidone (NMP), or the like.

[0107] The step of placing the separator 7 between the positive electrode 6 and the negative electrode 8 may include a step of manufacturing the separator 7 (separator membranes 7A, 7B, 7C). In this case, after obtaining the separator 7, the positive electrode 6, the separator 7 (separator membranes 7A, 7B, 7C) obtained by the above method, and the negative electrode 8 are stacked together using, for example, a laminate. This produces an electrode assembly 2 comprising the positive electrode 6, the negative electrode 8, and the separator 7 disposed between the positive electrode 6 and the negative electrode 8. This electrode assembly 2 can be housed in a battery housing 3 to obtain a lithium-ion secondary battery 1.

[0108] Example

[0109] Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to these Examples.

[0110] [Example 1]

[0111] A silicone rubber frame (4 x 4 cm, 1 mm thick) was placed on a PET sheet (8 x 8 cm, 0.035 mm thick). A slurry containing 6.0 g of polyethylene glycol diacrylate represented by the following formula (3) (where n = 14, product name: NKEster A-600, manufactured by SHIN-NAKAMURA CHEMICAL CO., LTD.), 4.0 g of diethylene glycol dimethyl ether (manufactured by FUJIFILM Wako Pure Chemical Corporation), 1.4 g of lithium nitrate (manufactured by FUJIFILM Wako Pure Chemical Corporation), and 1.0 g of azobisisobutyronitrile (manufactured by FUJIFILM Wako Pure Chemical Corporation) was placed in the frame. The polyethylene glycol diacrylate was polymerized by heating at 60°C for 1 hour using a hot plate to obtain a separation membrane made of poly(ethylene glycol diacrylate). The separation membrane was removed from the frame and used in the following test.

[0112]

[0113] [Example 2]

[0114] In Example 1, a separation membrane was prepared by the same method as in Example 1, except that polyethylene glycol diacrylate was replaced with polytetrafluoroethylene (manufactured by FUJIFILM Wako Pure Chemical Corporation) and azobisisobutyronitrile (manufactured by FUJIFILM Wako Pure Chemical Corporation) was not added. Thus, a separation membrane made of polytetrafluoroethylene (PTFE) was obtained.

[0115] [Example 3]

[0116] A porous body made of polyolefin (Gurley value: 1 sec / 100 cc, porosity: 60 vol%, thickness: 20 μm) was prepared. A slurry prepared in the same manner as in Example 1 was applied to the entire surface of the porous body, followed by heating at 60°C for 1 hour to polymerize the polyethylene glycol diacrylate. This yielded a separation membrane coated with poly(ethylene glycol diacrylate) in the porous body.

[0117] [Example 4]

[0118] In Example 3, a separation membrane was produced by the same method as in Example 3, except that polyethylene glycol diacrylate was replaced with the same polytetrafluoroethylene as in Example 2. Thus, a separation membrane in which PTFE was coated on a porous body was obtained.

[0119] [Example 5]

[0120] In Example 3, inorganic oxide particles Li7La3Zr2O were further added in an amount of 1 vol% relative to the total volume of the separation membrane. 12 A slurry of (manufactured by FUJIFILM Wako Pure Chemical Corporation) was applied to the porous body, and a separation membrane was prepared by the same method as in Example 3. Thus, a separation membrane was obtained in which a porous body made of a lithium ion conductive polymer and poly(ethylene glycol diacrylate) and inorganic oxide particles were coated.

[0121] [Example 6]

[0122] In Example 5, a separation membrane was prepared by the same method as in Example 5, except that polyethylene glycol diacrylate was replaced with tetrafluoroethylene, the same as in Example 2. Thus, a separation membrane was obtained in which PTFE and SiO2 were coated on a porous body made of a lithium ion conductive polymer.

[0123] [Comparative Example 1]

[0124] In Example 1, a separation membrane was produced by the same method as in Example 1, except that polyethylene glycol diacrylate was replaced with ethyl cyanoacrylate (manufactured by FUJIFILM Wako Pure Chemical Corporation). Thus, a separation membrane made of polyethyl cyanoacrylate was obtained.

[0125] [Comparative Example 2]

[0126] In Example 3, a separation membrane was prepared by the same method as in Comparative Example 1, except that polyethylene glycol diacrylate was replaced with ethyl cyanoacrylate as in Comparative Example 1. Thus, a separation membrane was obtained in which a polymer porous body having lithium ion conductivity was coated with polyethyl cyanoacrylate.

[0127] [Comparative Example 3]

[0128] In Example 5, a separation membrane was prepared by the same method as in Example 5, except that polyethylene glycol diacrylate was replaced with ethyl cyanoacrylate as in Comparative Example 1. Thus, a separation membrane was obtained in which a polymer porous body having lithium ion conductivity was coated with polyethyl cyanoacrylate and inorganic oxide particles.

[0129] <Evaluation of Solvent Separation Ability>

[0130] The separation membrane and separator (UP3085, manufactured by Ube Industries, Ltd.) involved in the embodiment or comparative example are overlapped, and they are sandwiched between two sheets of silicone rubber (thickness 0.5 mm) and arranged between H-type units. Dimethyl carbonate (DMC) is placed in the unit on the separation membrane side, and the appearance of the separator after a specified number of days is visually observed. If the solvent separation ability of the separation membrane is excellent, DMC is not easy to pass through the separation membrane, so DMC is not easy to penetrate into the separator, but if the separation membrane has a poor solvent separation ability, DMC penetrates the separation membrane and penetrates into the separator. Therefore, the separation ability of the solvent (equivalent to the solvent of the first solvent and the second solvent) of the separation membrane can be evaluated by observing the appearance of the separator and confirming whether DMC has penetrated into the separator. If DMC has not penetrated into the separator after 1 day from the start of the test, it is indicated as "≥1 day" in Tables 1 to 3. In this case, the solvent separation ability of the separation membrane can be said to be excellent. On the other hand, in Tables 1 to 3, cases where DMC crossover was not observed after one day are indicated as "<1 day." As shown in Tables 1 to 3, in the separation membranes according to the Examples, DMC did not cross over into the separator even after one day or more, whereas in the separation membranes according to the Comparative Examples, DMC crossover into the separator after one day.

[0131] [Table 1]

[0132]

[0133] [Table 2]

[0134]

[0135] [Table 3]

[0136]

[0137] Explanation of symbols

[0138] 1-Lithium-ion secondary battery, 2-Electrode group, 3-Battery casing, 4-Positive electrode collector sheet, 5-Negative electrode collector sheet, 6-Positive electrode, 7, 7A, 7B, 7C-Separator membrane, 8-Negative electrode, 9-Positive electrode collector, 10-Positive electrode mixture layer, 11-Negative electrode collector, 12-Negative electrode mixture layer, 21-Porous body, 22-Resin.

Claims

1. A lithium-ion secondary battery comprising a positive electrode mixture layer, a separator, and a negative electrode mixture layer in this order, wherein: The positive electrode mixture layer is composed of a positive electrode active material, a first lithium salt, a first solvent, 10% by weight or less of a binder, and 10% by weight or less of a conductive material. The negative electrode mixture layer is composed of a negative electrode active material, a second lithium salt, a second solvent different from the first solvent, 10% by weight or less of a binder, and 10% by weight or less of a conductive material. The separation membrane contains at least one resin selected from the group consisting of: (i) an acrylic resin containing as a monomer unit at least one of monomers having a (meth)acryloyl group represented by the following formula (1), and (ii) a fluororesin consisting solely of an olefin containing fluorine as a monomer unit; The content of the resin is greater than 40% by volume based on the total amount of the separation membrane; In formula (1), R 1 represents a hydrogen atom or a methyl group, and * represents a bond.

2. The lithium ion secondary battery according to claim 1, wherein The separation membrane includes a porous body formed of a polymer different from the resin and the resin held in the porous body, the porous body has a Gurley value of 20 seconds / 100 cc or less, and the porosity of the porous body is 30% by volume or more based on the volume of the porous body.

3. The lithium ion secondary battery according to claim 2, wherein The porous body has a Gurley value of 10 seconds / 100 cc or less, and a porosity of 40% by volume or more based on the volume of the porous body.

4. The lithium ion secondary battery according to claim 2 or 3, wherein The separation membrane further includes inorganic oxide particles held by the porous body.

5. The lithium ion secondary battery according to claim 1, wherein The separation membrane further contains a third lithium salt and a third solvent.

6. The lithium ion secondary battery according to claim 1, wherein The fluororesin is PTFE.

7. The lithium ion secondary battery according to claim 1, wherein The acrylic resin includes polyalkyl (meth)acrylate, poly (polyalkylene glycol di (meth)acrylate) or poly (meth)acrylic acid.

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